The present invention relates generally to infrared radiation display systems, and more particularly, to microbolometers for the detection of infrared radiation and operating systems therefor.
Infrared radiation display systems including microbolometers have found use in a wide variety of applications. Examples of applications include preventive maintenance of machinery, predictive maintenance of electrical power transmission equipment, fire prevention, fire fighting, energy conservation, law enforcement, navigation, security, and military applications.
A typical microbolometer array includes a number of thermally isolated sensor elements or pixels arranged into rows and columns. The sensor elements are typically thin film resistors that have a relatively high temperature coefficient. When infrared radiation strikes the sensor elements, the temperature of the thin film resistors rise, and the resistance changes. By measuring the resistance change, a measure of the incident infrared radiation can be determined.
The sensor elements are typically formed on a substrate using conventional semiconductor processing techniques. Each sensor element is typically provided on a bridge or the like that is thermally isolated from the substrate. Without significant thermal isolation from the substrate, the thermal mass of the substrate can prevent the incoming infrared radiation from significantly heating the thin film resistors of the sensor elements, which in turn, can significantly reduce the sensitivity of the sensor elements.
For some microbolometer arrays, one terminal of each thin film resistor is connected to a power supply voltage such as VDD. The other terminal of each thin film resistor is connected to a corresponding row read line through a column select switch. Each of the row read lines are connected to a readout line via a corresponding row select switch. The readout line can be coupled to a second supply such as ground or virtual ground through a transimpedance amplifier. The transimpedance amplifier typically provides an output signal that is related to the amount of incoming infrared radiation that is detected.
In operation, those sensor elements that are in a selected row are read in succession from a first sensor element to a last sensor element before the sensor elements in a next row are read. To accomplish this, a first row select switch is typically activated to connect a first row read line to the readout line. Then, the column select switches are sequentially activated to sequentially connect each of the sensor elements in the selected row to the selected row read line and thus the readout line. Once all sensor elements in a selected row are read, a next row select switch is activated to connect the next row read line to the read output line. Then, the column select switches are again sequentially activated to sequentially connect each of the sensor elements in the next row to the selected row read line and thus the readout line. This is continued for each row in the microbolometer array.
When the first row of sensor elements is being read, the row read lines of the unselected rows are disconnected from the readout line. Also, the column select switches that connect the unselected row read lines to VDD through the unselected sensor elements are being sequentially activated, which can selectively connect the unselected row read lines to VDD. Accordingly, the unselected row read lines tend to be at about VDD when they are unselected. When an unselected row read line becomes selected by a corresponding row select switch, the row read line is immediately connected to ground or virtual ground through the transimpedance amplifier. Because of the limited current that can be provided by the transimpedance amplifier, and because of the capacitance and resistance of the row read lines, the transition from one row read line to another can cause a transient on the newly selected row read line. This transient can make it difficult to get an accurate reading of the resistance of the first few sensor elements in each row. Thus, for some prior art microbolometer arrays, the readings for the first few columns of sensor elements may be inaccurate.
In addition, the transient current on the newly selected row read line can cause significant heating of the first few sensor elements in each row. Because the sensor elements are typically thermally isolated from the substrate, this heating can cause damage to the thin film resistors of the sensor elements.
What would be desirable, therefore, is a microbolometer array and/or operating system that reduces the possibility that a transient will occur on the row read line while a thermally isolated sensor element is read.
The present invention provides a microbolometer array and operating system that reduces the possibility that a transient will occur on a row read line while a thermally isolated sensor element is read. In one illustrative embodiment, one or more columns of xe2x80x9cdummyxe2x80x9d sensor elements are placed in the microbolometer array. The xe2x80x9cdummyxe2x80x9d sensor elements are preferably, but not necessarily, thermally connected to the substrate, allowing them to more readily handle the transient current spikes experienced by a newly selected row read line. The one or more columns of xe2x80x9cdummyxe2x80x9d sensor elements are preferably configured so that they are selected when a row read line is initially selected. Thus, it is the xe2x80x9cdummyxe2x80x9d sensor elements that are subjected to the transients on the newly selected row read line, and not the thermally isolated sensor elements that are used for imaging.
It is contemplated that a sufficient number of columns of xe2x80x9cdummyxe2x80x9d sensor elements may be provided so that the transients have sufficient time to settle out before the actual thermally isolated sensor elements are read. Alternatively, a single column of dummy sensor elements may be provided, and the control of the microbolometer array may be configured so that the dummy sensor element remains selected until the transients on the newly selected row read line have settled out.
In another illustrative embodiment, dummy sensors are not required. Instead, the control of the microbolometer array is configured to deselect all sensor elements in the array until the transients on the newly selected row read line have settled out. This can be accomplished by deselecting all column select switches for a period of time after a new row read line is selected. This may help prevent damage to the thermally isolated sensor elements.